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Commit4efaf4b

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Fix wrong logic in TransactionIdInRecentPast()
The TransactionIdInRecentPast() should return false for all the transactionsolder than TransamVariables->oldestClogXid. However, the function containsa bug in comparison FullTransactionId to TransactionID allowing fulltransactions between nextXid - 2^32 and oldestClogXid - 2^31.This commit fixes TransactionIdInRecentPast() by turning the oldestClogXid intoFullTransactionId first, then performing the comparison.Backpatch to all supported versions.Reported-by: Egor ChindyaskinBug: 18212Discussion:https://postgr.es/m/18212-547307f8adf57262%40postgresql.orgAuthor: Karina LitskevichReviewed-by: Kyotaro HoriguchiBackpatch-through: 12
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‎src/backend/utils/adt/xid8funcs.c

Lines changed: 19 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -90,11 +90,12 @@ typedef struct
9090
staticbool
9191
TransactionIdInRecentPast(FullTransactionIdfxid,TransactionId*extracted_xid)
9292
{
93-
uint32xid_epoch=EpochFromFullTransactionId(fxid);
9493
TransactionIdxid=XidFromFullTransactionId(fxid);
9594
uint32now_epoch;
9695
TransactionIdnow_epoch_next_xid;
9796
FullTransactionIdnow_fullxid;
97+
TransactionIdoldest_xid;
98+
FullTransactionIdoldest_fxid;
9899

99100
now_fullxid=ReadNextFullTransactionId();
100101
now_epoch_next_xid=XidFromFullTransactionId(now_fullxid);
@@ -128,17 +129,24 @@ TransactionIdInRecentPast(FullTransactionId fxid, TransactionId *extracted_xid)
128129
Assert(LWLockHeldByMe(XactTruncationLock));
129130

130131
/*
131-
* If the transaction ID has wrapped around, it's definitely too old to
132-
* determine the commit status. Otherwise, we can compare it to
133-
* ShmemVariableCache->oldestClogXid to determine whether the relevant
134-
* CLOG entry is guaranteed to still exist.
132+
* If fxid is not older than ShmemVariableCache->oldestClogXid, the
133+
* relevant CLOG entry is guaranteed to still exist. Convert
134+
* ShmemVariableCache->oldestClogXid into a FullTransactionId to compare
135+
* it with fxid. Determine the right epoch knowing that oldest_fxid
136+
* shouldn't be more than 2^31 older than now_fullxid.
135137
*/
136-
if (xid_epoch+1<now_epoch
137-
|| (xid_epoch+1==now_epoch&&xid<now_epoch_next_xid)
138-
||TransactionIdPrecedes(xid,ShmemVariableCache->oldestClogXid))
139-
return false;
140-
141-
return true;
138+
oldest_xid=ShmemVariableCache->oldestClogXid;
139+
Assert(TransactionIdPrecedesOrEquals(oldest_xid,now_epoch_next_xid));
140+
if (oldest_xid <=now_epoch_next_xid)
141+
{
142+
oldest_fxid=FullTransactionIdFromEpochAndXid(now_epoch,oldest_xid);
143+
}
144+
else
145+
{
146+
Assert(now_epoch>0);
147+
oldest_fxid=FullTransactionIdFromEpochAndXid(now_epoch-1,oldest_xid);
148+
}
149+
return !FullTransactionIdPrecedes(fxid,oldest_fxid);
142150
}
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/*

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